This invention relates to the formation of pellets for use in a melting furnace and, more particularly, to the formation of pellets for use in a glass melting furnace.
Copending U.S. application Ser. No. 965,632 now U.S. Pat. No. 4,212,613, Seng, filed on Dec. 1, 1978, discloses the formation of batch material into pellets by adding the batch material and liquid, such as water, to a rotary pelletizer. Rotation of the pelletizer mixes the batch material with the liquid such that the batch material added to the pelletizer evolves from a batch appearance to non-adhering discernible nuclei or seeds which gradually grow as they are moved around the pelletizer by the pelletizer's motion. The finished pellets may be placed directly into a melter or supplied to a heat exchanger through which hot gases from either the melter or an external source are passed. Preferably, the pellets are dried and preheated in accordance with the teachings of Hohman, Seng, Henry and Propster in U.S. Pat. Nos. 4,248,615 and 4,248,616. The preheating of the pellets provides an energy savings since less energy is required to melt the batch. In addition, the hot exhaust gases from the furnace contain useful batch particulates that are imparted to the batch, thereby allowing recovery of this material which would otherwise be expelled into the atmosphere.
The components of the batch material supplied to the pelletizer tend to segregate during transportation thereto. This segregation is not harmful to the operation of a glass furnace, since the components of the pellets average out over a period of time. However, the short variations in the batch components affect the pellet forming ability of the batch material, and hence the size of the finished pellets. Certain glass batches have particle sizes that vary widely, thus resulting in greater segregation and variation of the batch than for other batches. In addition, the feed rate of the batch to the pelletizer may vary. Therefore, it is necessary to accurately predict the size of the pellets being formed within the pelletizer so that the water being supplied to the pelletizer can be controlled to respond to variations in the composition of the batch and/or the rate of feed of the batch, thereby producing pellets within the desired tolerance. As is known in the art, increasing the amount of water supplied to the pelletizer increases the size of the finished pellets; whereas, decreasing the amount of water reduces the size of the finished pellets.
The pellets should be uniform within a prescribed tolerance. Generally, pellets having a nominal diameter of one-half inch, with a range of three-eighths to five-eighths inch, have been found to be the optimum size for obtaining maximum heat transfer from the hot combustion gases to the pellets. If the pellets are too small, they excessively restrict the flow of gases through the preheater; whereas, if the pellets are too large, the surface to weight ratio is reduced which results in less heat being transferred to the pellets. Furthermore, the large pellets may have moisture trapped therein which may cause them to explode when the moisture turns to steam.
A rotary pelletizer can be divided into contiguous sections with each formative stage of the pellets pertaining to a respective section of the pelletizer. Generally, the lighter pellets, which are in the earliest formative stages, will be propelled in the widest rotational path and will follow an elliptical path with the widest circular orbit. As the pellets become heavier, the motion or path of the pellets will become more elliptical, since the heavier pellets will experience a deceleration sooner than the lighter pellets. Accordingly, it is possible to identify the formative stages as well as the relative sizes of the pellets on a cross section of the pelletizer, by identifying the appropriate sector in that cross section.
Seng, U.S. Pat. No. 4,212,613, discloses the use of a pelletizer having a rotating disc, a sensor for detecting variations in the batch level and means operatively connected to the sensor for varying the amount of water supplied to the pelletizer in response to the sensed batch level. The sensor is positioned generally at an upward portion of the disc where it is contacted during the pelletization process by an upwardly moving stream of batch material prior to the formation of non-adhering discernible pellet seeds. For certain glasses, namely low alkali-alkaline earth-aluminoborosilicate glasses, this arrangement has worked quite well, providing a shortened control loop for controlling the size of the finished pellets.
Typically, the low alkali-alkaline earth-aluminoborosilicate glasses contain alkali metal oxides in an amount that is less than 3% by weight and, more typically, less than 1% by weight, if at all. Generally, the cumulative amount of silica, alumina, alkaline earth metal oxides and boric oxide will comprise at least about 85% by weight and, more commonly, on the order of about 93-95% by weight of such glasses. The typical batch ingredients which are admixed prior to pelletization are sand, limestone, clay and a calcium borate, such as, calcined colemanite. An example of such glasses is that commonly referred to in the art as an E-type textile glass.
In contrast to such low alkali-alkaline earth-aluminoborosilicate glasses, it has been found that the location of the sensor disclosed in Seng does not produce the degree of desired sensitivity and control in pelletizing other glass batches. Exemplary of such glasses where the desired sensitivity and control of the pellet forming means is not obtained, are the soda containing glasses, for example, those glasses in which Na.sub.2 O is present in an excess of about 5% by weight and, more typically, in an amount of about 10-20% by weight. Such glasses are represented by the common soda lime silica glasses, in which the cumulative amount of silica, calcium oxide and sodium oxide are in excess of about 60% by weight, preferably in excess of about 75% by weight and more preferably in excess of about 90% by weight of the glass, and by alkali-alkaline earth-aluminoborosilicates.
Therefore, it is an object of this invention to provide a method of and apparatus for pelletizing a batch of particulate material for such alkali-alkaline earth-aluminoborosilicate glass.